The present invention generally relates to an image reading apparatus, and more particularly to an image reading apparatus provided with a projector unit for projecting an image from an original image on a translucent film. This apparatus is applicable to a copying machine, a film reader, an electronic filing system, a facsimile machine and the like.
An image reading apparatus provided with a projector unit for projecting an image from an original image on a translucent film (including a negative image film and a positive image film) has been devised in order to read out an image signal by scanning such an original image under appropriate light quantity conditions. For example, a microfilm reader (or a microfilm reader/printer) provided with a projector for projecting an image from a translucent microfilm by emitting light to the microfilm, is one of the above described apparatus. Such a device always maintains the quantity of light in the projector unit at a constant level during an image reading process.
However, when an image signal is read out by an image reader from an original image, it is necessary that different light sources be used for two kinds of original images: a reflection type image (for example, a document image) and a translucent type image. The illuminances of images projected from these two kinds of original images, and the spectral transmittance distributions of the transmitting light are considerably changed according to the light source used. Thus, if the light quantity of the light source for only the translucent film type image is fixed or maintained at a constant level, it is necessary to use two different image processing circuits dealing with image signals at separate levels, which signals are obtained from the two kinds of original images. Therefore, the light quantity of the projector lamp in the above described apparatus cannot be accurately adjusted to suitably control the light quantity, and there is a problem in that the size of the circuitry in the above apparatus becomes relatively large, thus increasing its manufacturing cost.
Japanese Laid-Open Patent Application No. 63-292121 discloses an image projector device using light quantity adjustment. In this device, an image is projected onto an image reader part at an appropriate illuminance, the image reading part being made from a number of charge coupled devices (CCDs). According to the method of this reference, electric voltage applied to a lamp of a projector unit is adjusted according to an image signal obtained by the CCD unit from the projected image so that an image is projected at an appropriate illuminance of the light emitted by the projector lamp. However, the voltage applied to the projector lamp is adjusted so as to obtain a desired light quantity from the projector lamp, and therefore the color temperature of the lamp is considerably changed and the spectral transmittance distribution of the transmitting light is excessively changed. Especially when a color image is read out from a color original image, it is difficult to accurately adjust a shading data because the ratio of the levels of color signals obtained from the original image is considerably changed. In some cases, undesired shading takes place in the reproduced color image, and the color of the original image is incorrectly turned into a different color.
Moreover, Japanese Laid-Open Patent Application No. 2-100459 discloses another one of the above described apparatus. In the apparatus of this reference, an image signal is obtained by a CCD reading part from a projected image, and a difference between a light quantity corresponding to the image signal and a predetermined light quantity is calculated and this calculated difference is output to a display device. By viewing the displayed difference, a user can manually adjust the light quantity emitted by a projector unit. However, there still remains the above mentioned problem due to the variations of the voltage applied to the lamp. The light quantity must be manually adjusted by the user, which adjustment work requires additional time and labor. In addition, the manual adjustment by the user is likely to produce large variations of the light quantity of the light source.